Finger couplers or DC blocks are utilized for the direct voltage decoupling of components in HF technology, in particular in radar technology. These elements are part of the strip line circuit and are thus etched as a structure. The HF signal passes through a bandpass characteristic due to the overlapping fingers; the direct voltage, however, is blocked. This bandpass characteristic is essential for the function of the radar, since it prevents low-frequency portions of the control pulse from being relayed. Therefore, a single series capacitance is not sufficient.
The finger width of a coupler is a function of the substrate and the conductor impedance. The common conductor impedance is a standard 50 Ohm and is determined by the conductor width. Considering the given parameters, one would arrive at a finger width of 90 μm and a gap of 60 μm. These dimensions cannot be implemented in a large-scale production process. In order to nevertheless implement this coupler, as FIG. 2 shows, upstream and downstream of coupler 1, a line transformation 2 to a lower, complex impedance is performed, which has the effect that the finger width increases to 200 μm and the gap increases to 120 μm which makes them suitable for manufacturing, as FIG. 2 shows.